Re: C++ PATCH for c++/91678 - wrong error with decltype and location wrapper

[Marek Polacek <polacek@redhat.com>]

Ping.

On Thu, Sep 05, 2019 at 10:24:55PM -0400, Marek Polacek wrote:
> Compiling this testcase results in a bogus "invalid cast" error; this occurs
> since the introduction of location wrappers in finish_id_expression.
> 
> Here we are parsing the decltype expression via cp_parser_decltype_expr which
> can lead to calling various fold_* and c-family routines.  They use
> non_lvalue_loc, but that won't create a NON_LVALUE_EXPR wrapper around a location
> wrapper.
> 
> So before the location wrappers addition cp_parser_decltype_expr would return
> NON_LVALUE_EXPR <c>.  Now it returns VIEW_CONVERT_EXPR<float *>(c), but the
> STRIP_ANY_LOCATION_WRAPPER immediately following it strips the location wrapper,
> and suddenly we don't know whether we have an lvalue anymore.  And that's sad
> because then decltype produces the wrong type, causing nonsense errors.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk and 9?
> 
> 2019-09-05  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/91678 - wrong error with decltype and location wrapper.
> 	* parser.c (cp_parser_decltype): Use auto_suppress_location_wrappers
> 	sentinel.  Don't strip location wrappers.
> 
> 	* g++.dg/cpp0x/decltype73.C: New test.
> 
> diff --git gcc/cp/parser.c gcc/cp/parser.c
> index baa60b8834e..b3c7bff5988 100644
> --- gcc/cp/parser.c
> +++ gcc/cp/parser.c
> @@ -14729,8 +14729,13 @@ cp_parser_decltype (cp_parser *parser)
>        /* Do not warn about problems with the expression.  */
>        ++c_inhibit_evaluation_warnings;
>  
> +      /* Don't create wrapper nodes within decltype.  non_lvalue_loc won't
> +	 create a NON_LVALUE_EXPR wrapper around a location wrapper, and a
> +	 subsequent STRIP_ANY_LOCATION_WRAPPER would destroy the information
> +	 about lvalueness of the expression.  */
> +      auto_suppress_location_wrappers sentinel;
> +
>        expr = cp_parser_decltype_expr (parser, id_expression_or_member_access_p);
> -      STRIP_ANY_LOCATION_WRAPPER (expr);
>  
>        /* Go back to evaluating expressions.  */
>        --cp_unevaluated_operand;
> diff --git gcc/testsuite/g++.dg/cpp0x/decltype73.C gcc/testsuite/g++.dg/cpp0x/decltype73.C
> new file mode 100644
> index 00000000000..cbe94a898e3
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp0x/decltype73.C
> @@ -0,0 +1,4 @@
> +// PR c++/91678 - wrong error with decltype and location wrapper.
> +// { dg-do compile { target c++11 } }
> +
> +float* test(float* c) { return (decltype(c + 0))(float*)c; }